Embodiments of the inventive concept disclosed herein relate to a semiconductor memory device, and more particularly, to a variable resistance memory device having a bidirectional switch, a memory system including the same, and an operating method thereof.
Semiconductor memory devices are classified into a volatile semiconductor memory device and a nonvolatile semiconductor memory device. The volatile semiconductor memory device is fast in terms of read and write speeds, but it loses data stored therein when power is not supplied thereto. In contrast, the nonvolatile semiconductor memory device retains data stored therein even when power is not supplied thereto. For this reason, the nonvolatile semiconductor memory device is used to store information that has to be retained regardless of whether power is supplied thereto.
There is an increasing demand for a semiconductor memory that is randomly accessible and implements high integration and a large capacity. A typical example of the semiconductor memory device is a flash memory that is mainly used in a portable electronic device currently. In addition, there is being developed a semiconductor memory device in which a DRAM capacitor is replaced with a nonvolatile material. Next-generation memory devices that are in the spotlight while satisfying the above-described requirements include a phase change random access memory (PRAM), a nano floating gate memory (NFGM), a polymer RAM (PoRAM), a magnetic RAM (MRAM), a ferroelectric RAM (FeRAM), a resistive RAM (RRAM), etc.
In the case of the PRAM, unselected word lines are precharged in advance even in a stand-by state to support a high access speed. In addition, a bidirectional switch (or diode) is being used to form a PRAM cell of a three-dimensional structure. When the bidirectional switch is used, a word line has to be precharged with a higher voltage than when a unidirectional switch is used. In this case, a bit line may be precharged with the same voltage level as that of the word line to prevent current leakage. Accordingly, in a highly-integrated PRAM, it is necessary to prevent a potential leakage current due to precharging of the word line and the bit line.